The broad objective of this proposal is to study the regulation of erythroid progenitor cell differentiation. To this end, the biochemical nature, the mechanism of action, the mechanism of elaboration and the physiological significance of two erythroid burst promoting activities will be investigated. One of these factors, code named RP, has been isolated from protein concentrates of the urine of anemic patients, the other is elaborated, in response to phytohemagglutinin stimulation, by a nonadherent, non SRBC-rosetting subset of human blood mononuclear cells in culture. In order to quantitate these factors, assays for them will be developed, based on their ability to increase the number of erythropoietin stimulated erythroid burst colonies derived from mouse bone marrow cells and from human blood mononuclear cells respectively. The factors will be purified by techniques including selective membrane ultrafiltration, fractional precipitation and various types of chromatography. The specificity of the factors, at various stages of purification, will be tested by observing their effects in the exhypoxic polycythemic mouse assay and in various hematopoietic cell clonogenic assays. Prior to challenge with erythropoietin, target cells will be preincubated for various lengths of time with different amounts of factors to determine the reversibility time course and dose dependence of their action. Cell separation procedures will be employed to produce cell fractions enriched in certain cells from the respective target populations, to investigate the role they play in the process leading to an increased number of erythroid burst forming units. It is intended to measure the RP content of the urine of patients in various anemic and polycythemic states and of normal individuals and also to characterize their blood mononuclear cells as to their ability to produce, as well as to respond to, burst promoting activity after PHA treatment in order to discern whether these measurements provide clinically useful information.